From spit system to tide-dominated delta: integrated reservoir model of the upper Jurassic Sognefjord Formation on the Troll West Field

The late Jurassic Sognefjord Formation is the main reservoir unit on the giant Troll West field, and its sedimentology and stratigraphy are discussed in an accompanying paper (Dreyer et al., this volume). The Sognefjord Formation was deposited in a shallow-marine setting characterized by wave-, tide- and fluvial depositional processes. Sediments were brought into the area mainly from the NE, and initially distributed by strong longshore currents downdrift from the mouth-bar area. The coastal morphology was that of an elongated spit sandbar up to 40 km in length. Seaward of the spit, a moderately steep shoreface existed (see cores from 31/2-18), characterized by wave- and storm-related processes (see cores from 31/2-F-6 H). Pronounced clinoforms extending from the spit platform (topsets) to the offshore transition (bottomsets) are clearly visible on seismic data. Landward of the spit, a major embayment existed, in which fluvial and tidal processes were dominant. The distribution of reservoir sandstones reflects this morphology, with the high reservoir quality units mainly corresponding to the spit sandbar. At the transition between the middle and late Oxfordian, a marked relative sealevel fall created an extensive unconformity (see cores 31/2-18 and 31/5-H-5 H) and led to a major change in coastal morphology. The sediments from this upper part of the Sognefjord Formation were formed in a tide-dominated deltaic setting, and in general have a more heterolithic character (see cores 31/2-18 and 31/5-H-5 H). Estuarine sandbars, tidal channels and sandy portions of tidal flat represent the main oil-bearing targets.